Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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  #1   Report Post  
Andres
 
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Default Variac Help. Trying to find the right one.

I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum). I've seen very
high prices for power supplies at some vendors, and I read that a
VARIAC can be a good, and cheap choice. Now, what is a variac? Can it
be used to evaporate gold, silver, aluminum? Can I have a variac to
control 3 boats? Not at the same time but one after the other? Any
help is appreciated.

A confused undergrad
  #3   Report Post  
DoN. Nichols
 
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Default Variac Help. Trying to find the right one.

In article ,
Andres wrote:
I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum). I've seen very
high prices for power supplies at some vendors, and I read that a
VARIAC can be a good, and cheap choice. Now, what is a variac?


It is one brand of a variable autotransformer. And since it is
a brand, it is spelled "Variac" -- not all upper case or all lower case.
The maker -- General Radio -- I believe to be out of business now, but I
could be wrong. Another maker is Superior Electric (and they call their
model a "Powerstat" instead).

A variable autotransformer can produce output from 0V to either
full line voltage or somewhere above full -- e.g. a 120V line with 140V
full scale on a tapped Variac. I have even seen center-tapped ones
which could take a 120VAC input and produce 240V output (at reduced
current, of course).

Can it
be used to evaporate gold, silver, aluminum?


Not by itself. Even for the very big ones, the voltage is too
high, and the current is too low -- and boats tend to be very low
resistance devices, so they call for low voltage and high current.

However, it can be used to adjust the voltage into a step-down
transformer whose secondary is designed for the desired current, and can
go to perhaps 50% over the needed voltage for the boats.

Can I have a variac to
control 3 boats? Not at the same time but one after the other?


Yes -- with some way to switch the very high current selectively
into the boat of your desire. For infrequent changes, you could unbolt
the wires from the feedthrough terminals for one boat, and connect them
to another boat.

For more frequent changes, I would suggest high-current knife
switches. The ones which I have seen used had blades of about 1/4"
thickness and perhaps 1-1/4" width. They will last longer if you turn
the Variac down to 0V output before switching.

Any
help is appreciated.

A confused undergrad


You have what I know about the subject -- from working with
Variacs and other variable autotransformers, and observing various sized
vacuum evaporation systems in use where I used to work.

Good Luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
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  #4   Report Post  
william_b_noble
 
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Default Variac Help. Trying to find the right one.

perhaps if you offered some details on power supply specs, I could help -
there are plenty of sources of high current power supplies, but without some
clue as to what you actually need, it is hard to help. Do you need AC or
DC, do you need a specific voltage? do you need regulation of any kind?
since I'm not familiar with this specific field, I presume a "boat" is not
an aquatic device for carrying passangers but some kind of smaller thing
with metal in it -- are you heating it with current directly, or via a
resistance element? do you have a budget?

Maybe someone already knows all the answers for you, but I'd be happy to
help if you can clarify the question.
"Andres" wrote in message
om...
I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum). I've seen very
high prices for power supplies at some vendors, and I read that a
VARIAC can be a good, and cheap choice. Now, what is a variac? Can it
be used to evaporate gold, silver, aluminum? Can I have a variac to
control 3 boats? Not at the same time but one after the other? Any
help is appreciated.

A confused undergrad



  #5   Report Post  
Gary Coffman
 
Posts: n/a
Default Variac Help. Trying to find the right one.

On 10 Mar 2004 20:40:18 -0800, (Andres) wrote:
I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum). I've seen very
high prices for power supplies at some vendors, and I read that a
VARIAC can be a good, and cheap choice. Now, what is a variac? Can it
be used to evaporate gold, silver, aluminum? Can I have a variac to
control 3 boats? Not at the same time but one after the other? Any
help is appreciated.


A variac is just an autotransfromer with a mechanical sliding tap.
By turning a knob, you change the tap point, hence the output
voltage. Variacs over about 500 watts tend to be expensive.
Used carelessly, they can also easily be damaged (burnt contact
finger, smoked windings, etc). They are best used as set and
forget devices. Adjusting them under heavy load tends to rapidly
burn the contact finger.

I don't know how precise temperature control needs to be for
your application, nor how many watts of heat you need. But I'd
guess that you'd be better served by an electronic phase angle
type of control, using some sort of feedback to maintain set
point temperature.

If the set point temperature doesn't have to be tightly held,
then a simple bang bang type control (either all the way on
or all the way off, switching according to measured temperature
feedback) will be all you'll need. If the allowable deviation from
set point temperature is small, then a proportional type control
will be required.

But if your wattage requirements are modest, and manual
adjustment is sufficient, then an ordinary lamp dimmer control
might be all you need. This would have the same user interface
as a Variac, ie a knob, but at the same power levels it would
cost 10 to 20 times less.

Gary


  #6   Report Post  
John D. Farr
 
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Default Variac Help. Trying to find the right one.

There is about always several variacs on eBay. gl, John

"Gary Coffman" wrote in message
...
On 10 Mar 2004 20:40:18 -0800, (Andres) wrote:
I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum). I've seen very
high prices for power supplies at some vendors, and I read that a
VARIAC can be a good, and cheap choice. Now, what is a variac? Can it
be used to evaporate gold, silver, aluminum? Can I have a variac to
control 3 boats? Not at the same time but one after the other? Any
help is appreciated.


A variac is just an autotransfromer with a mechanical sliding tap.
By turning a knob, you change the tap point, hence the output
voltage. Variacs over about 500 watts tend to be expensive.
Used carelessly, they can also easily be damaged (burnt contact
finger, smoked windings, etc). They are best used as set and
forget devices. Adjusting them under heavy load tends to rapidly
burn the contact finger.

I don't know how precise temperature control needs to be for
your application, nor how many watts of heat you need. But I'd
guess that you'd be better served by an electronic phase angle
type of control, using some sort of feedback to maintain set
point temperature.

If the set point temperature doesn't have to be tightly held,
then a simple bang bang type control (either all the way on
or all the way off, switching according to measured temperature
feedback) will be all you'll need. If the allowable deviation from
set point temperature is small, then a proportional type control
will be required.

But if your wattage requirements are modest, and manual
adjustment is sufficient, then an ordinary lamp dimmer control
might be all you need. This would have the same user interface
as a Variac, ie a knob, but at the same power levels it would
cost 10 to 20 times less.

Gary



  #7   Report Post  
Wayne
 
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Default Variac Help. Trying to find the right one.

I use one to cut foam. I use a thin wire then appy some current until it is
glowing then you just feed the foam , like a band saw. I have only used it a
few times but it works very nice. It also "seals" the edge of the foam as it
melts it.


  #8   Report Post  
Ed Huntress
 
Posts: n/a
Default Variac Help. Trying to find the right one.

"Wayne" makowicki wrote in message
...
I use one to cut foam. I use a thin wire then appy some current until it

is
glowing then you just feed the foam , like a band saw. I have only used it

a
few times but it works very nice. It also "seals" the edge of the foam as

it
melts it.



Watch out for fumes. Styrene fumes are bad. Polyurethane fumes are seriously
toxic. Don't use a hot wire with polyurethane foam.

Ed Huntress


  #9   Report Post  
jim rozen
 
Posts: n/a
Default Variac Help. Trying to find the right one.

In article , Andres says...

I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum).


You need to have a high current transformer to do what you
want to do.

A very small alumina coated tungsten boat will take about
50 amps or so to get any decent rate. So the typical
routine for an inexpensive thermal evaporator is to
use a transformer that has a 120 volt primary and a
100 amp or so secondary. The transformer's primary
is then run off a variac to control the boat current.

Because the boat's resistance is a strong function of
temperature you need to ride the variac to keep the
rate more or less constant.

The easiest way to switch between sources is to ground
one end of each, and ground one end of the transformer
secondary. Then use a large aligator clip on the other
transformer lead to connect to the other lead of each boat
in turn.

Companies like Denton used to use us huge mercury switches
to switch between two sources.

You can net a bunch of parts if you can find an old Veeco
or Denton system. I myself has a tiny lab evaporator
made from a scrap denton that has been retrofitted with
a balzers turbopump. But it still has the original variac,
transformer, bell jar, and baseplate.

Jim

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please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

  #10   Report Post  
william_b_noble
 
Posts: n/a
Default Variac Help. Trying to find the right one.

if all you need is 50 to 100 amps, and IF the voltage is modest, look for an
old computer power supply (not a desktop computer) - it's not hard to find a
300 amp 5V supply surplus for $15 or so


"jim rozen" wrote in message
...
In article , Andres

says...

I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum).


You need to have a high current transformer to do what you
want to do.

A very small alumina coated tungsten boat will take about
50 amps or so to get any decent rate. So the typical
routine for an inexpensive thermal evaporator is to
use a transformer that has a 120 volt primary and a
100 amp or so secondary. The transformer's primary
is then run off a variac to control the boat current.

Because the boat's resistance is a strong function of
temperature you need to ride the variac to keep the
rate more or less constant.

The easiest way to switch between sources is to ground
one end of each, and ground one end of the transformer
secondary. Then use a large aligator clip on the other
transformer lead to connect to the other lead of each boat
in turn.

Companies like Denton used to use us huge mercury switches
to switch between two sources.

You can net a bunch of parts if you can find an old Veeco
or Denton system. I myself has a tiny lab evaporator
made from a scrap denton that has been retrofitted with
a balzers turbopump. But it still has the original variac,
transformer, bell jar, and baseplate.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================





  #11   Report Post  
jim rozen
 
Posts: n/a
Default Variac Help. Trying to find the right one.

In article m, william_b_noble
says...

if all you need is 50 to 100 amps, and IF the voltage is modest, look for an
old computer power supply (not a desktop computer) - it's not hard to find a
300 amp 5V supply surplus for $15 or so


The problem with this approach is the variable resistance of the
boat as it comes up to temperature. Because a computer supply is
probably a regulated switching type supply, it will be tough
to ramp the temperature of the thermal source in a smooth fashion.

There are really only two ways to do this, manually with a variac
on the primary of a high current transformer, or using a true
regulated current source supply that has a variable output. It
can also be done with a light dimmer on the primary of a transformer.

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

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